The proposed research centers around two recently described immunoglobulin allotypes, d11 and d12. Genetic and structural studies revealed that these markers represent another group of specificities associated with the heavy chains of the IgG molecules. Our immediate objectives are, 1) to demonstrate the precise point of interaction between the group d allotypic determinants and the specific alloantibody, 2) to evaluate the distribution of the group d antigens in the wild Australian rabbit population, particularly with reference to the other associated heavy chain allotypes, group a and group e, and 3) to examine the role of immunogenic RNA in conferring the constant region allotypic specificities to RNA transformed lymphoid cells.